8
FRIEDHELM HILDEBRANDT
Tertiary structure of DNA
If a piece of double stranded DNA is unwound, the tertiary structure of a
superhelixwill result. (An overwound telephone cord may serve as an example of a superhelix from common experience.) In Figure 4A negative
supercoiling is demonstrated in the example of a bacterial plasmid, a circular piece of double stranded DNA, which in its relaxed form forms a circle. If the plasmid is cut, the resulting linear piece of DNA is unwound and
the two ends are then rejoined covalently, the energetically favoured conformation that results is a right-handed superhelix or a left-handed spiral
will result. Most naturally occurring DNA adopts this negatively-supercoiled state. Now, if only one strand is cut, the negative supercoiling is dissipated and a relaxed circle ofDNA re-forms. Electron micrographs ofthese
structures are shown in Figure 4B.
RNA structure
The most important structural differences between DNA and RNA are emphasized in Table 2. RNA (and single stranded DNA) can spontaneously
form an energetically more stable secondary structure via base paring within the same single stranded nucleic acid molecule. Figure 4C shows an example of "hairpin formation", which can occur in nucleotide sequences exhibiting dyad symmetry. Dyad symmetry of a sequence is present if a sequence is followed by its reverse complement ( e. g.: AACGG-CCGTT). Hairpin formation can play a role as a transcription terminator in prokaryotes
(Figure 4C). Transfer RNA undergoes extensive secondary structure formation to form an L-shaped molecule (Figure 4D).
Table 2. Some features distinguishing DNA from RNA
Most abundant natural
occurrence
Sugar
Nucleotides
Chemical stability
DNA
double-stranded
2' -deoxyribose
A, C, G, and T
relatively stable at room
temperature
RNA
single-stranded
ribose
A, C, G, and U
(also uncommon bases)
relatively unstable at room
temperature (hydrolysed by
RNases, alkali)
FRIEDHELM HILDEBRANDT
Tertiary structure of DNA
If a piece of double stranded DNA is unwound, the tertiary structure of a
superhelixwill result. (An overwound telephone cord may serve as an example of a superhelix from common experience.) In Figure 4A negative
supercoiling is demonstrated in the example of a bacterial plasmid, a circular piece of double stranded DNA, which in its relaxed form forms a circle. If the plasmid is cut, the resulting linear piece of DNA is unwound and
the two ends are then rejoined covalently, the energetically favoured conformation that results is a right-handed superhelix or a left-handed spiral
will result. Most naturally occurring DNA adopts this negatively-supercoiled state. Now, if only one strand is cut, the negative supercoiling is dissipated and a relaxed circle ofDNA re-forms. Electron micrographs ofthese
structures are shown in Figure 4B.
RNA structure
The most important structural differences between DNA and RNA are emphasized in Table 2. RNA (and single stranded DNA) can spontaneously
form an energetically more stable secondary structure via base paring within the same single stranded nucleic acid molecule. Figure 4C shows an example of "hairpin formation", which can occur in nucleotide sequences exhibiting dyad symmetry. Dyad symmetry of a sequence is present if a sequence is followed by its reverse complement ( e. g.: AACGG-CCGTT). Hairpin formation can play a role as a transcription terminator in prokaryotes
(Figure 4C). Transfer RNA undergoes extensive secondary structure formation to form an L-shaped molecule (Figure 4D).
Table 2. Some features distinguishing DNA from RNA
Most abundant natural
occurrence
Sugar
Nucleotides
Chemical stability
DNA
double-stranded
2' -deoxyribose
A, C, G, and T
relatively stable at room
temperature
RNA
single-stranded
ribose
A, C, G, and U
(also uncommon bases)
relatively unstable at room
temperature (hydrolysed by
RNases, alkali)